1. Field of the Invention
The present invention relates to a charged-particle beam lithographic system.
2. Description of Related Art
A charged-particle beam lithographic system is equipment for writing ultrafine patterns such as semiconductor integrated circuit patterns on substrates such as semiconductor substrates and mask blanks using a charged particle beam such as an electron beam.
A variable-shaped (variable-area) charged-particle beam lithographic system is disclosed as one type of charged-particle beam lithographic system, for example, in JP-A-2007-43078. The operation of this disclosed system is described. An electron beam emitted from an electron gun illuminates a whole first aperture having a square hole via an illumination lens. First, the electron beam is shaped into a square form. The electron beam of the first aperture image passed through the first aperture is projected onto a second aperture via a projector lens. The position of the first aperture image on the second aperture is controlled by a deflector. The shape and dimensions of the beam can be varied. The electron beam of the second aperture image passed through the second aperture is focused by an objective lens, deflected by another deflector, and projected at a desired position on a sample lying on an X-Y stage that is disposed movably. In this case, it follows that the illuminating electron beam is rectangularly shaped in conformity with the pattern shape using a shot size less than a set maximum shot size.
A mask blank to be lithographically written is generally configured including a glass base, a light-shielding film layer made of Cr or other material and formed on the glass base, and a resist layer formed on the light-shielding film layer. This mask blank is mounted on a stage using a clamping mechanism or the like within a charged-particle beam lithographic system. Lithography is performed using an electron beam. During this lithography, if a part of the electron beam hits the side surface of the mask blank, then the glass base will be electrostatically charged. This in turn deviates the orbit of the electron beam.
Accordingly, a charged-particle beam lithographic system set forth in JP-A-2008-58809 uses a substrate cover including a frame-like member that is so sized that its outer periphery is located outside the outer periphery of a substrate. The frame-like member has a central opening having dimensions smaller than the outer size of the substrate. This cover shields the side surface of the substrate from the charged particle beam.
The substrate cover set forth in patent document 2 is mounted on a substrate. When the substrate is exchanged, the substrate cover is conveyed into the system together with the substrate and mounted on a stage. After the end of a lithography operation, the substrate cover is removed from the stage together with the substrate and taken out of the system. However, the substrate cover has an outer size greater than the outside dimension of the substrate and, therefore, it is difficult to convey the substrate cover by a transport device such as a robot arm or to mount the cover onto the stage by the transport device. As a result, the productivity is deteriorated. A structure having a substrate cover that is not mounted on a substrate is also conceivable. In this case, the substrate is first conveyed and mounted on a stage. Then, the substrate cover is conveyed to overlap the substrate and mounted on the stage. After the end of a lithography operation, the substrate cover is first removed from the stage and then the substrate is removed and conveyed out of the system. Again, the substrate cover that is larger in size than the substrate must be conveyed using a transport device. The productivity is deteriorated for a similar reason.